Sedimentary Rocks: Formation and Classification

2. White Sands Nat’l Monument, NM Molly Rogers

3. Dolomite Mountains Lea Korsmeyer

4. SEDIMENTATION AND SEDIMENTARY ROCKS SEDIMENTARY STRUCTURES CROSS-BEDDING Consists of sedimentary layers deposited at an angle to the underlying set of beds. Navajo Sandstone Utah Coconino Sandstone Arizona

5. SEDIMENTATION AND SEDIMENTARY ROCKS SEDIMENTARY STRUCTURES CROSS-BEDDING Form when particles drop from a moving current, such as a flowing river or wind. Cross beds always slope towards the down-current direction. Consequently, they can be used to indicate direction of flow.

6. SEDIMENTATION AND SEDIMENTARY ROCKS SEDIMENTARY STRUCTURES CROSS-BEDDING

7. SEDIMENTATION AND SEDIMENTARY ROCKS SURFACE SEDIMENTARY STRUCTURES Surface layers of sedimentary rocks also have structures. RIPPLE MARKS Shallow, curving ridges produced by currents

8. SEDIMENTATION AND SEDIMENTARY ROCKS SURFACE SEDIMENTARY STRUCTURES RIPPLE MARKS

9. SEDIMENTATION AND SEDIMENTARY ROCKS SURFACE SEDIMENTARY STRUCTURES MUD CRACKS Polygonal cracks that form as mud is exposed to air and then dries out. Occurs when lakes dry up or evaporate.

10. SEDIMENTATION AND SEDIMENTARY ROCKS LITHIFICATION When sediments are buried they retain heat. Some heat comes from buried radioactive grains. Some heat conducted from Earth’s interior. Sediments are also compressed by overlying sediments. Sediments are invaded by groundwater carrying ions. Heat, pressure and ions alter the physical and chemical nature of the sediments by a processes collectively known as DIAGENESIS.

11. SEDIMENTATION AND SEDIMENTARY ROCKS LITHIFICATION DIAGENESIS can lead to LITHIFICATION. LITHIFICATION is the conversion of loose sediments to solid sedimentary rocks. Usually occurs in the upper few kilometers of crust. Temperatures are <200C and at low pressure. Sediments are COMPACTED and often CEMENTED.

12. SEDIMENTATION AND SEDIMENTARY ROCKS LITHIFICATION COMPACTION - the process by which pressure reduces the volume of sediment during diagenesis. Overlying sediment squeezes air and water out of sediments forcing grains closer together. Clays have weak attractive forces which causes grains to adhere and become SEDIMENTARY ROCK.

13. SEDIMENTATION AND SEDIMENTARY ROCKS LITHIFICATION CEMENTATION takes place as water circulates between grains and compounds originally dissolved during chemical weathering are precipitated and bind sediments together. Types of cement: Calcium carbonate (CaCO3) Silica (SiO2) Iron oxides (Fe2O3, FeOOH) Iron sulfide (FeS2) Iron carbonate (FeCO3) Organic fragments are also compacted and cemented.

14. SEDIMENTATION AND SEDIMENTARY ROCKS LITHIFICATION Increased heat and pressure can also cause RECRYSTALLIZATION. Recrystallization is the development of stable minerals from unstable varieties. Many marine organisms secrete aragonite in skeletons. Aragonite recrystallizes to calcite upon the death of the organism that produced it. This explains why there is no aragonite in ancient carbonate rocks.

15. SEDIMENTATION AND SEDIMENTARY ROCKS CLASSIFICATION OF SEDIMENTARY ROCKS Grain-size forms the basis of clastic sedimentary rock classification. Sediment Size (mm) Particle Name Rock Name >256 Boulder 64-256 Cobble Conglomerate Breccia Gravel 4-64 Pebble Granule 2-4 0.063-2 Sand Sandstone 0.004-0.063 Silt Siltstone Mudstone Mud <0.004 Clay Shale

16. SEDIMENTATION AND SEDIMENTARY ROCKS CLASSIFICATION OF SEDIMENTARY ROCKS CONGLOMERATES AND BRECCIAS Both have particles sizes > 2mm. Conglomerates have rounded grains. Indicates greater weathering and erosion. Breccias have angular grains. Indicates less weathering and erosion.

17. SEDIMENTATION AND SEDIMENTARY ROCKS CLASSIFICATION OF SEDIMENTARY ROCKS SANDSTONES Clastic sedimentary rocks with a grain-size between 0.063 (1/16) and 2 mm. Represent 25% of all sedimentary rocks. Are usually cemented by carbonate (calcite) or silica. >90% quartz grains = quartz sandstone well sorted and rounded grains >25% feldspar = arkose poorly sorted and angular grains >33 1/3 silt, clay and rock fragments = greywacke poorly sorted and angular grains

18. SEDIMENTATION AND SEDIMENTARY ROCKS CLASSIFICATION OF SEDIMENTARY ROCKS SANDSTONES

19. SEDIMENTATION AND SEDIMENTARY ROCKS CLASSIFICATION OF SEDIMENTARY ROCKS SANDSTONES Sandstones are useful as building stones contain oil, gas and groundwater due to high porosity and permeability

20. SEDIMENTATION AND SEDIMENTARY ROCKS CLASSIFICATION OF SEDIMENTARY ROCKS MUDSTONES Grain-size is small, < 0.063 (1/16) mm. Mudstones with particles <0.004 mm in diameter constitute >50% of all clastic sedimentary rocks. Usually extremely fine-grained. Settle out in very quiet waters. Lakes, lagoons, floodplains and ocean floors. Generally composed of micas and clay minerals. These can be oriented parallel to each other.

21. SEDIMENTATION AND SEDIMENTARY ROCKS CLASSIFICATION OF SEDIMENTARY ROCKS MUDSTONES Parallel alignment of micas and clays is termed FISSILITY. SHALE = FISSILE mudstone.

22. SEDIMENTATION AND SEDIMENTARY ROCKS CLASSIFICATION OF SEDIMENTARY ROCKS MUDSTONES Color in MUDSTONES is variable. Color usually depends on depositional environment. Black = High organic content Low oxygen content Red = Oxidizing environment Terrestrial settings Green = Reducing environment Marine settings with low O2